Conductive connections, structures with such connections, and methods of manufacture

ABSTRACT

In some embodiments, to increase the height-to-pitch ratio of a solder connection that connects different structures with one or more solder balls, only a portion of a solder ball&#39;s surface is melted when the connection is formed on one structure and/or when the connection is being attached to another structure. In some embodiments, non-solder balls are joined by an intermediate solder ball ( 140   i ). A solder connection may be surrounded by a solder locking layer ( 1210 ) and may be recessed in a hole ( 1230 ) in that layer. Other features are also provided.

BACKGROUND OF THE INVENTION

The present disclosure relates to conductive connections such as solder connections. Such connections are used, for example, in packaging of semiconductor integrated circuits, and in particular in attaching an integrated circuit or a packaging substrate to another integrated circuit or substrate. Examples of packaging substrates include wiring substrates (e.g. printed circuit boards) and interposers.

Solder is widely used in such connections because, for example, a solder connection can be easily undone (by melting the solder) to repair or replace a defective component without replacing the entire package.

FIGS. 1A and 1B illustrate solder attachment of an integrated circuit 110A to a structure 110B. Structure 110B may be another integrated circuit, or a printed circuit board (PCB), or some other packaging substrate, or can be an integrated circuit package. Circuit 110A has electrical contacts (“contact pads”) 120A which must be soldered directly to contact pads 120B of structure 110B. Such direct solder attachment is called “flip-chip” attachment. (Alternatively, contact pads can be interconnected by discrete wires, but flip-chip attachments are preferred because they reduce the assembly size and they lower the electrical resistance, parasitic capacitance, and parasitic inductance.)

As shown in FIG. 1A, solder balls 140 are formed on pads 120A. (The solder balls may alternatively be formed on pads 120B or on both pads 120A and 120B.) Then the structures 110A and 120B are brought together so that the solder 140 physically contacts the contact pads 120A and 120B. This assembly is placed into an oven to melt the solder. The solder then cools and re-solidifies to attach the pads 120A to the corresponding pads 120B.

During manufacture and subsequent circuit operation, solder connections 140 can be pulled sideways by various forces. A common source of such forces is thermal expansion: structures 110A, 110B may expand or contract due to heating or cooling, and the structure 110A may expand or contract by a different amount than the structure 110B if the two structures have different coefficients of thermal expansion (CTE). Solder connections 140 may crack or break, impairing or destroying the electrical functionality. Reliability of solder connections is thus an important goal in designing a manufacturing process.

Solder connections can be made more reliable by increasing their height H. However, increasing the height also increases the solder ball's width W because the solder tends to become spherical when melted. As a result, adjacent pads 120A must be spaced farther apart so as not to be shorted to each other. The minimal distance between pads 120B is also increased. Of note, the distance between the centers of adjacent pads 120 cannot be smaller than the distance between the centers of the adjacent solder balls, i.e. the solder ball pitch; the pitch is therefore greater than or equal to W. The increased pitch undesirably increases the assembly size and may degrade the electrical functionality (for example, by requiring longer interconnect lines (not shown) and/or by making the circuit slower and/or more power-hungry).

Thus, it is desirable to increase the height H without the corresponding increase in W, i.e. to increase the ratio H/W (this ratio is about equal to the height-to-pitch ratio because the solder ball width W is about equal to the minimal solder ball pitch). In order to increase this ratio, a solder connection can be made as a stack of solder balls. FIG. 2 illustrates a stack 210 of four solder balls 140.1, 140.2, 140.3, 140.4 on a contact pad 120 of a structure 110. The stack can be produced by a solder jetting apparatus 220 of type SB²-Jet available from Pac Tech (Packaging Technologies USA, Inc.). See e.g. E. Zakel et al., “High Speed Laser Solder Jetting Technology for Optoelectronics and MEMS Packaging”, ICEP 2002, incorporated herein by reference. In this apparatus, melted or at least softened solder balls 140 are supplied one after another through a nozzle 240. Gas pressure is used to propel and place each solder ball on top of contact pad 120 or of another, previously placed solder ball. As each solder ball is being placed, the solder ball is melted by a laser source 250 and is then cooled to adhere to the contact pad 120 or the underlying solder ball. Solder stack 210 does indeed have a higher H/W ratio than a single solder ball, but the H/W increase is limited because the solder balls 140 are squashed (flattened) in the jetting process. The flattening acts to increase W and decrease H. For example, if each solder ball 140 has the same volume as a sphere of radius 250 μm, then the stack of two solder balls 140.1, 140.2 may have the height H of only 400 μm or less, not 500 μm. The solder ball width W is higher than 250 μm, so the height-to-pitch ratio is smaller than 400/250=1.6 for the two-ball stack, or smaller than 0.8 per ball.

After solder stack formation, the structure 110 is attached to another structure (as in FIG. 1B). In this structure-to-structure attachment, the solder stack is reflowed (melted), and the ratio H/W may further decrease during reflow. For example, a 450 μm diameter solder ball may become only 360 μm high, requiring a pad-to-pad pitch of about 600 μm (unless the solder is laterally restrained by a mask, not shown). The entire solder stack may even collapse. To counteract this phenomenon, one of the balls 140 can be made of a high-melting-temperature material having a higher melting temperature than the other balls so as not to melt during the structure-to-structure attachment. Such a technique is described in U.S. Pat. No. 6,455,785 issued Sep. 24, 2002 to Sakurai et al. In that technique, the bottom balls 140.1 are made of gold, and the overlying balls of indium (indium has a lower melting temperature than gold). Each ball is formed by heating a tip of a metal wire (gold or indium wire, heated by a high voltage discharge or a gas flame): the molten tip forms a ball which is placed on the structure, and the wire is then cut off. A wire residue remains as a small protrusion on top of the ball, and these protrusions are flattened by a plate pressing on the balls. Then, in the structure-to-structure attachment, only the indium balls are reflowed, the gold ball remains solid to prevent collapse.

FIGS. 3A, 3B illustrates another bumping technique from a PhD Dissertation by Xingshen Liu entitled “Processing and Reliability Assessment of Solder Joint Interconnection for Power Chips”, PhD Dissertation, 2001 (URN etd-04082001-204805), Virginia Tech Digital Library, Chapter II, incorporated herein by reference. First, solder balls 140.1 are formed on contacts 120A from solder paste. Then higher-melting-temperature balls 140.2 are placed on balls 140.1, and solder 140.1 is reflowed. Balls 140.2 are never melted. Solder balls 140.3 are formed on contact pads 120B of another structure 110B from solder paste. Solder 140.3 has lower melting temperature than 140.1. Then structure 110A is placed on structure 110B, and the solder 140.3 is reflowed for attachment to solder 140.2. Solders 140.1 and 140.2 are not melted in this process.

Further improvements are desirable in forming conductive connections.

SUMMARY

This section summarizes some of the exemplary implementations of the invention.

In some embodiments for forming a solder connection, only a portion of the solder ball's surface is melted. The height-to-pitch ratio is therefore increased in some embodiments, or at least is less likely to decrease. For example, in some embodiments, the height-to-width ratio H/W is greater than 0.8 per solder ball, and can be 1 or close to 1. This technique can be used to form solder stacks, but can also be used to form single-ball connections (i.e. connections having only one solder ball as in FIG. 1B).

A solder connection may include non-solder components, e.g. solder-coated balls having a non-solder core which does not melt at any stage (the core can be a high-melting-temperature metal and/or a polymer and/or an insulator).

In some embodiments, a solder or non-solder connection (e.g. a copper post) is recessed at the top in a surrounding dielectric layer in a structure 110A. Another structure (e.g. 110B) has a protruding conductive post which is inserted into the recess and bonded to the connection. The recess helps align the connection of structure 110A with the post of structure 110B. Also, if solder is used, the recess restrained the solder and thus helps to avoid an electrical short with nearby connections. Restraining the solder also strengthens the bond between the connection and the post because if the solder is not restrained then it flows away and is wasted. Other embodiments and variations are within the scope of the invention as defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 2, 3A, 3B show vertical cross sections of structures during and after fabrication according to prior art.

FIGS. 4A, 4B, 4C, 5A, 5B, 6A, 6B, 7, 8, 9, 10, 11A, 11B, 12A, 12B, 13, 14A, 14B, 14C, 15, 16A, 16B, 17A, 17B, 18, 19A, 19B, 20, 21A, 21B, 21C, 21D, 22, 23 show vertical cross sections of structures during and after fabrication according to certain embodiments set forth in further detail below.

DESCRIPTION OF SOME EMBODIMENTS

The embodiments described in this section illustrate but do not limit the invention. The invention is defined by the appended claims.

FIGS. 4A, 4B illustrate attachment of a solder ball 140 to contact pad 120 according to some embodiments. Structure 110 can be an integrated circuit, an interposer, a rigid or flexible wiring substrate, and/or of any type described above for structures 110, 110A, 110B, and/or a combination of the types described above (e.g. a package). Contact pad 120 is shown at the top surface of structure 110, but can be at any surface: the invention is not limited to any special orientation. Solder 140 is melted only at its bottom region 510. The rest of solder 140 is not melted during the attachment, so the solder ball's height is more stable. The melting can be performed by heat supplied by electromagnetic radiation, for example by a laser pencil 250. In some embodiments, laser 250 emits a pulse of infrared radiation (e.g. 1064 nm wavelength) of 1 μsec to a few milliseconds in duration, but the invention is not limited to any pulse durations, wavelengths, or other parameters. However, the pulse and other parameters are chosen so as to not to melt the rest of the solder ball. Suitable parameters depend on the solder material, the size of region 510, and possibly other factors, and such parameters can be experimentally determined without undue experimentation. See for example U.S. Pat. No. 5,977,512 issued Nov. 2, 1999 to Azdasht et al. describing laser parameters for melting an entire solder ball; this patent is incorporated herein by reference. In some embodiments, the laser beam is focused by appropriate optics. Focused or unfocused non-laser light can also be used. The light beam can arrive at area 510 from a side as in FIG. 4A, or from another direction; in some embodiments, the light source is positioned below the structure 110, and the light beam travels through the structure to region 510. The invention is not limited to a particular source of heating energy, or a position of the source, or other parameters.

In some embodiments, the melted region 510 is at most 50% of the volume of solder 140 (the volume is taken before melting). In some embodiments, region 510 is 1% to 50% of the entire volume. In some of these embodiments, the solder ball is between 5 μm and 1 mm in diameter (the diameter is the maximum pre-melted dimension: the solder ball may or may not be perfectly round). For example, the diameter can be from 25 μm to 500 μm, possibly not more than 250 μm, possibly not more than 175 μm. These ranges are not limiting. In some embodiments, region 510 is a surface region whose depth is below 50%, or even below 25%, or below 10% of the solder ball's pre-melted or melted or post-melted diameter or height.

Solder ball 140 is placed on contact pad 120 so that the region 510 is in physical contact with the pad. The heating energy is supplied to region 510 before and/or after the solder ball placement. Then region 510 is cooled and re-solidified to fuse the solder ball to the contact pad. Contact pad 120 is not melted in the fusing process. The placing of solder 140 on pad 120 can be effected by any suitable means, possibly using conventional methods such as solder jetting, or such as forcing the solder 140 through a stencil's opening aligned with pad 120 (the solder could be forced by a squeegee, a piston, gas, possibly just by its own weight, or other suitable force). Other placement methods can also be used.

In some embodiments, in the fusing process, some of the molten or softened solder is squeezed out due to the weight of solder 140 and/or due to additional pressure that may be applied to the solder ball and/or the contact pad (e.g. by gas or squeegee etc.). The “squeeze-out” region 520 (FIG. 4B) protrudes laterally outward at the junction of solder 140 and contact pad 120. In some embodiments, region 520 is a continuous protrusion that protrudes laterally on all sides of solder connection 140. Region 520 may or may not be perfectly round.

FIG. 4C is an enlarged view of solder 140. The shape of squeeze-out 520 depends on the size and temperature of region 510, the pressure applied to ball 140, and possibly other parameters. In some embodiments, the pre-melted height Hsq of squeeze-out 520 is about 1% to 50% of the height H of solder 140. In some embodiments, the depth of region 510 is less than 50% of the solder ball diameter, so Hsq is less than 50% of H. The maximum squeeze-out width SqWmax is from 1% to 150% of the width Wb of the solder portion above the squeeze-out. The solder width SqWmin at the top end of the squeeze-out is from 1% to 100% of the non-squeeze-out solder width Wb.

As noted above, the invention is not limited to any special orientation. For example, at any stage of the attachment process or subsequently, solder 140 can be below the contact pad or in some other orientation.

In FIGS. 5A and 5B, solder 140 has been attached to contact pad 120A of structure 110A, and is being attached to pad 120B of structure 110B to electrically connect the pads together. The attachment to pad 120A was made by the method of FIGS. 4A-4B (the squeeze-out is shown at 520A), but other methods including prior art methods could be used instead. As shown in FIG. 5A, the solder ball's region 510B opposite to pad 120A is melted without melting the rest of the solder. Structures 110A and 110B are arranged so that the region 510B physically contacts the pad 120B. The heating energy (e.g. laser or other types described above) is supplied to region 510B before and/or after the region 510B is placed on pad 120B. Region 510B is cooled and re-solidified to form the attachment. Squeeze-out 520B can be formed at the solder junction with pad 120B. Squeeze-out 520B may have the same shape (continuous lateral protrusion on all sides) and dimensions as given above for squeeze-out 520 of FIGS. 4B-4C. Squeeze-out 520B may or may not have the same shape and dimensions as squeeze-out 520A.

In the example of FIGS. 6A, 6B, the partial melting (i.e. melting less than all the solder of a solder ball) is used to form a solder stack. First, solder ball 140.1 is attached to pad 120 of a structure (not shown) such as described above in connection with FIG. 4A. In this example, the attachment to pad 120 was made by the method of FIGS. 4A-4B (the squeeze-out is shown at 520.1), but prior art methods could be used instead. Then solder ball's region 510 facing away from pad 120 is melted without melting the rest of the solder. Unmelted solder ball 140.2 is placed on ball 140.1 so as to physically contact the region 510. Solder ball 140.2 is never melted except possibly for a trifling amount melted by the heat transfer from ball 140.1. The heating energy is supplied to region 510 before and/or after the placement of solder ball 140.2, by any heating source described above (e.g. laser or non-laser electromagnetic source). Then region 510 is cooled and solidified to form the attachment. Squeeze-out 520.2 (FIG. 6B) can be formed at the junction of the two solder balls. Squeeze-out 520.2 may or may not have the same shape and dimensions as given above for squeeze-out 520 of FIGS. 4B-4C and may be irregular or regular in nature. Squeeze-out 520.2 may or may not have the same shape and dimensions as squeeze-out 520.1.

FIG. 7 shows a similar technique (partial melting) but the melting is performed on solder 140.2. The melted region 510 forms part of solder 140.2, the rest of solder 140.2 is unmelted. Solder 140.1 is also unmelted, except possibly for a trifling amount melted by heat transfer from ball 140.2. The ball 140.2 is placed so that its region 510 physically contacts the ball 140.1. The heating energy is supplied before and/or after the placement of ball 140.2 on ball 140.1 by any heating source described above. When solder re-solidifies, the structure is as in FIG. 6B.

FIG. 8 shows a similar technique but with both solder balls 140.1, 140.2 being partially melted. The melted regions are shown respectively at 510.1, 510.2; the remaining solder is not melted. The heating energy is supplied before and/or after the placement of ball 140.2 on ball 140.1. When solder re-solidifies, the structure is as in FIG. 6B.

In FIG. 9, the attachment is formed using an intermediate solder ball 140 i which is melted in its entirety (by any suitable heating source described above in connection with FIG. 4A). Solder balls 140.1 are 140.2 are unmelted except possibly for trifling amounts melted by heat transfer from the intermediate ball. In some embodiments, the pre-melted size (diameter, or maximum dimension) and/or height of ball 140 i is 1% to 100% of the smallest of the pre-melted sizes and/or heights of balls 140.1, 140.2; and/or the pre-melted volume of ball 140 i is 1% to 100% of the smallest of the pre-melted volumes of balls 140.1, 140.2. These dimensions are not limiting. In some embodiments, ball 140 i is melted only once during the attachment of ball 140.2 to 140.1. For example, in some embodiments, the melting heat is supplied to ball 140 i before and/or after the placement of ball 140 i on ball 140.1; while the ball 140 i is still melted, the second ball 140.2 is placed on ball 140 i. Then ball 140 i is solidified by cooling. The final structure can be as in FIG. 6B, with the squeeze-out 520.2 formed by intermediate ball 140 i.

In other embodiments, intermediate ball 140 i is melted when attached to lower ball 140.1, then the intermediate ball is solidified, and then is melted again to attach ball 140.2. The balls 140.1, 140.2 are not melted except possibly at and/or near the junction with ball 140 i as noted above.

The method of FIG. 9 can be used to connect a solder ball 140 to a contact pad, as illustrated schematically in FIG. 10 (solder ball 140 is connected to pad 120A by intermediate ball 140 i.1, and then connected to pad 120B by intermediate ball 140 i.2; the final structure is as in FIG. 5B).

Any one or more of solder balls 140, 140 i may have a non-solder central core 140C (FIGS. 11A, 11B) coated by solder shell 140S. FIGS. 11A and 11B illustrate the use of such balls in the process of FIGS. 6A, 6B. After the attachment (FIG. 11B), the cores 140C may or may not touch each other. The cores 140C are not generally melted, i.e. regions 510 are generally restricted to solder shells 140S; if such a ball is used as an intermediate ball 140 i (FIGS. 9-10), only the solder shell 140S is melted (possibly the entire shell is melted). Core 140C does not significantly melt in this process; for example, core 140C can be a metal (e.g. copper or nickel) having a higher melting temperature than shell 140S, or the core 140C can be a polymer or an insulating material, e.g. polyimide; while the shell 140S can be a material having a melting temperature below 400° C. (e.g. a tin-based solder or indium), and so forth.

The methods described above can be used to form a conductive connection as a stack of any number of balls. The methods described above can be combined with other methods, e.g. prior art methods; for example, in the same stack, one or more balls can be attached by a method or methods according to some embodiments of the present invention, while one or more other balls can be attached by a prior art method or methods. The connection height is more stable. For example, if a solder stack is made using partial melting of solder balls, and the pre-melted height of each region 510 of each ball 140 is less than 50% of the height of the ball, then the stack height will be more than 50% of the sum of the pre-melted heights of the balls. Likewise, if a sequence of solder or non-solder balls 140 is attached to each other by intermediate solder balls 140 i as in FIG. 9, and there is one intermediate solder ball 140 i above (or below) each ball 140, and the pre-melted height of each intermediate ball 140 i is less than 50% of the sum of the heights of balls 140, then the stack height will be more than 50% of the pre-melted height. Further, if the pre-melted height of each region 510 or each intermediate ball 140 i is less than 25% of the heights of respective balls 140, then the final solder height is more than 75% of the pre-melted height. More generally, if the height of each region 510 or each ball 140 i is less than X % of the heights of respective balls 140, then the stack height will be more than (100−X) % of the pre-melted height.

Advantageously, in some embodiments, all the solder used in balls 140 (i.e. solder 140 or 140 i or 140S) in the entire stack can be made of the same material or of different materials having the same melting temperature. The melting regions are limited by limiting the spread of the electromagnetic radiation (e.g. by using laser and/or focused radiation). Using the same material may simplify the manufacturing process. The invention is not limited to such embodiments however.

Structure-to-structure attachments can be performed by a method according to the present invention (e.g. as in FIG. 5B or 10) or a prior art method, e.g. by solder reflow in an oven. Of note, there could be multiple contact pads 120A (possibly hundreds or thousands) that must be attached to corresponding pads 120B, and it may be desirable to form such attachments simultaneously when the tips of all the solder connections are in the same plane (we use the term “solder connections” to include connections with non-solder components, e.g. with non-solder cores 140C or with non-solder balls joined by intermediate solder balls 140 i). However, it may be inconvenient to perform simultaneous heating of multiple regions 510 or intermediate balls 140 i by limited-spread electromagnetic radiation as in FIGS. 4A-11B due to the large radiation power that would have to be simultaneously delivered to regions 510 or balls 140 i, possibly through one of the structures or through other intervening components (not shown). Therefore, in some embodiments, the structure-to-structure attachment is performed by solder reflow in an oven. To stabilize the solder height in this process, the solder can be locked in a solder locking layer 1210 (FIG. 12A) before the oven reflow. Solder locking layer 1210 (typically dielectric) exposes the tops of solder connections 140 but laterally surrounds and encapsulates the remaining solder of each solder connection so as to maintain the solder shape (and height) during the reflow at the stage of FIG. 12B. (In the example of FIGS. 12A-12B, the solder connections are as in FIG. 6B, but the technique can be used with other types of connections described above). Solder locking layer 1210 is formed at temperatures that do not allow the solder 140 to melt. In some embodiments, solder locking layer 1210 is a molded compound (i.e. a cured molding compound), typically dielectric, e.g. epoxy resin, formed by conventional techniques, e.g. transfer or compression molding, film-assist molding, or strip molding. Solder locking layer 1210 initially encapsulates the entire solder 140, and possibly (but not necessarily) physically contacts the solder 140 on all sides (all around); but later the layer 1210 is “shaved”, i.e. partially removed (e.g. by wet blasting using aluminum slurry or other chemicals, dry blasting, laser ablation, or other techniques) to expose the tops of solder stacks 210. See e.g. U.S. Pat. Nos. 6,838,637 and 6,635,209, both incorporated herein by reference.

During the structure-to-structure attachment, the solder is reflowed at a temperature that will not damage the solder locking layer 1210. For example, in some embodiments, the solder melting temperature is 217° C., the solder locking layer is a molded compound formed at temperatures not exceeding 120° C. to 200° C., and in the structure-to-structure attachment the solder is reflowed in an oven at 245° C. to 255° C. These examples are not limiting.

In some embodiments, solder locking layer 1210 includes a number of layers arranged to provide desired properties, e.g. a low combined dielectric constant. For example, in FIG. 13, solder locking layer 1210 includes layer 1210.1 and a later deposited molded compound 1210.2. In some embodiments, layer 1210.2 has a high dielectric constant (e.g. 4, 5, or greater); and layer 1210.1 has a lower dielectric constant (e.g. below 4, possibly below 3). Non-limiting examples of layer 1210.1 include silicon dioxide and parylene (parylene is described for example in U.S. Pat. No. 7,923,823 issued Apr. 12, 2011 to Mengel et al. and incorporated herein by reference; parylene's dielectric constant can be about 2.5.).

Initially, each of layers 1210.1, 1210.2 covers the entire structure including the solder stacks. Then both layers are shaved to expose the solder stacks 210 and provide the structure of FIG. 12A. In some embodiments, both layers 1210.1, 1210.2 are shaved in the same process (e.g. by wet blasting or other techniques described above).

In another example (FIG. 14A), layer 1210.2 is shaved first selectively to layer 1210.1 to expose the top of layer 1210.1. Then layer 1210.1 is removed over stacks 210 selectively to layer 1210.1 (FIG. 14B). This process forms holes 1230 in molded compound 1210.2 over the solder stacks over structure 110A. In other words, solder stacks 210 are recessed at the top surface of molded compound 1210.2. As shown, stack 210 extends through a through-hole in layer 1210 and physically contacts the stack 210; the top portion of this through-hole is the hole 1230 which has a concave sidewall 1232. The invention is not limited to any sidewall profile however.

The structure is subsequently attached to structure 110B (FIG. 14C). In the example shown, structure 110B is provided with protruding contact posts 1240 on contacts 120B. Each post may be a solder ball, a solder stack, a copper post, or solder on a copper post, or of some other type. Posts 1240 are inserted in holes 1230 before or during the solder reflow. Holes 1230 help in aligning the posts 1240 with connections 210. Also, holes 1230 restrain the melted solder during the reflow, and thus reduce the possibility of electrical shorts between laterally adjacent contacts 120. In some embodiments, the pitch between contacts 120 can be reduced as a result. (In some embodiments, the molded layer 1210.2 does not wet the solder to further help restrain the solder's lateral flow.)

In some embodiments, the depth Dh of each hole 1230 is 0.5 to 30 microns, and preferably 2 to 10 microns; and the diameter Wh (when viewed from the top in FIG. 14B, i.e. from the bottom in FIG. 14C) is 1 to 200 microns, and preferably 2 to 100 microns. “Diameter” is the largest lateral dimension at the top of each hole; the holes may or may not be circular. These particulars are not limiting.

In some embodiments, due to holes 1230, the assembly is tolerant to non-uniform heights of posts 1240 as long as the shortest posts 1240 can reach the connections 210: indeed, the longer posts 1240 will displace more solder 210 during solder reflow, but the excess solder can be accommodated in holes 1230 around the posts 1240 if the holes 1230 are sufficiently wider than the posts. For the same reason, the assembly is tolerant to non-uniform heights of connections 210.

In some embodiments, protruding posts 1240 are solder connections made by a method described above. For example, they can be formed as in FIG. 12A: see FIG. 15. Thus, the protruding stacks (as in FIG. 12A) and the recessed stacks (as in FIG. 14B) can be made by similar processes, simplifying the required tooling and the bill of materials. Posts 1240 can also be other type, including bond wires described in U.S. Pat. No. 8,618,659 issued Dec. 31, 2013 to Sato et al., incorporated herein by reference.

If desired, structure 110B can be bonded to solder locking layer 1210.2 in areas surrounding the holes 1230. The bonding can be by any suitable method, e.g. adhesive (not shown). Spacers (not shown) can be inserted between the structure 110B and layer 1210.2 in the bonding area, and/or adhesive can serve as spacers.

The recessed solder connections of FIG. 14B can be made even if each solder connection has only one solder ball. Also, solder-coated balls can be used (as in FIG. 11B).

Another technique to form recessed solder connections is to use a sublimating material that sublimes before or during solder reflow. In some embodiments, vaporizable materials may be used in place of the sublimating material. In another embodiment sublimating and vaporizable materials may be combined in appropriate concentration in conjunction with the solder particles. (As used herein, “sublimation” means direct transition from solid to vapor; “evaporation” or “vaporization” means transition to vapor through the liquid phase.) In FIG. 16A, a ball of such a material 1250 is placed on top of each solder stack 210 before formation of solder locking layer 1210. In some embodiments, the balls of material 1250 are placed by jetting, with complete or partial melting of balls of material 1250 and/or underlying solder. The material 1250 is heated for a very short time, to a temperature not sufficient to sublimate or evaporate the material 1250 in the placement operation. (The term “sublimate or evaporate” implies that either one or both of sublimation and evaporation may be present at the same time.)

In FIG. 16A, the ball of material 1250 on the right is spherical or almost spherical, but the ball of material 1250 on the left comes down around the sides of the solder stack 210 (around all sides in some embodiments). These details are not limiting.

Solder locking layer 1210 can be formed by any of the techniques described above or other techniques. In some embodiments, solder locking layer 1210 initially covers the balls of material 1250 but is later shaved to expose the balls of material 1250. In the embodiment shown, the shaved layer 1210 has a planar top surface but a non-planar surface is obtained in some embodiments. In some embodiments, layer 1210 has holes 1230 as in FIG. 14B, and have balls 1250 recessed in the respective holes, and these holes are enlarged by subsequent sublimation or evaporation of material 1250 as described below.

After exposure of the balls of material 1250, the balls of material 1250 are sublimated or evaporated in a heating operation to provide (or enlarge) the holes 1230 (FIG. 16B). The sublimation can be enhanced by performing the heating operation in vacuum environment (e.g. 70 kPa or even higher vacuum). Alternatively or in addition, the evaporation can be enhanced by elevated temperatures (e.g. above 150° C.; suitable temperatures may be lower if vacuum is used.) In some embodiments, the heating operation is performed in vacuum environment in reducing ambient. The solder may or may not be melted during the sublimation or evaporation step (i.e. the heating operation). In some embodiments, the heating operation is performed in an oven during or immediately before the structure-to-structure attachment (see FIG. 14C), i.e. during solder reflow. Sidewall 1232 of each hole 1230 may be concave; the sidewall completely laterally surrounds the connection. The invention is not limited to a particular sidewall profile.

Each hole 1230 is a top portion of the through-hole in which the layer 1210 physically contacts the connection 210. In some embodiments, each ball of material 1250 is 1 to 300 microns and preferably 3 to 100 microns in diameter; the underlying connections 210 may vary from 2 to 300 microns and preferably from 3 to 150 microns in diameter Wh (when viewed from the top); an exemplary depth Dh of each hole 1230 is 2 to 80% of the diameter of the corresponding connection 210, and the hole's diameter at the top is 5 to 80% of the diameter of the corresponding connection 210. In the left hole in FIG. 16B, the connection 210 protrudes upward at the bottom of hole 1230. In some embodiments, the height Ph of this upward protrusion is 0.1 to 50% of the height of the connection 210, and preferably 5 to 30%. These particulars are not limiting.

In some embodiments, the connections 210 are made of a non-solder material; e.g. they can be copper or nickel posts.

Material 1250 can be any material consistent with its desired use as described above. Thus, in some embodiments, material 1250 is solid at room temperature, and can be softened by heat when the ball of material 1250 is placed on the underlying surface (i.e. on connection 210) to reliably adhere to the top surface of connection 210 during the placement and even after cooling. Material 1250 can be chosen to also adhere well to native oxides if any that may form on the top surface of connection 210. In some embodiments, when the ball of material 1250 is being placed on connection 210, good adhesion between material 1250 and connection 210 is obtained by softening the material 1250 at a temperature below the melting temperature of the top surface of connection 210. In some embodiments, the softening can be performed by electromagnetic radiation, e.g. laser, that will not damage other parts of the structure. In some embodiments, the bottom portion of material 1250 is softened while the top portion is not softened. In some embodiments, the top portion of connection 210 is softened or melted to perform the placement.

Material 1250 can sublimate at suitable temperatures as described above. Exemplary sublimatable materials include waxes; high-molecular-weight polyethylene glycols (PEGs); polyethylene oxides (PEOs); and polyoxyethylene-polypropylene block copolymers. The molecular weights of these classes of materials may vary between 100 and 8000000. In some embodiments, the waxes and the PEG or PEO are combined. In another example, apart from paraffinic waxes, terpenoids may be used such as camphor 1,7,7-Trimethylbicyclo[2.2.1]heptan-2-one, camphor-10-sulfonic acid and their various analogues and related moieties may be used. These materials are suitable for many compositions of connections 210 including solder and non-solder compositions that include tin, lead, copper, nickel, silver, gold, platinum, and their combinations, and other materials.

FIGS. 17A and 17B illustrate another embodiment in which the material 1250 is included in each ball 140 in the stacks. More particularly, each ball 140 has the following structure: solder core 140C coated with material 1250. Exemplary thickness of coating material 1250 on each ball is 1 to 5 microns for the core 140C having any diameter described above for balls 140 and 140 i, but these dimensions are not limiting. The coating material 1250 can be formed by any suitable technique, e.g. by spraying the cores 140C with a liquid which, when cured, provides the coatings of material 1250; or by dipping the cores 140C into a non-solid substance which, when cured, provides the coatings of material 1250. (In other embodiments, only some of balls 140 have this structure, while the other balls 140 may be of the type described above in relation to FIGS. 1A-16B.) Alternatively, or in addition, material 1250 may be mixed with the solder (e.g. inside a metallurgical solder network) in any one or more (possibly all) of balls 140. The placement of each ball 140 may be performed using the techniques described above or other techniques. For example, the placement may be performed by jetting with partial or complete melting or softening of material 1250, possibly with squeeze-out portions at the junctions of balls 140; in some embodiments, the solder portions of balls 140 (e.g. solder cores 140C) are not melted during the ball placement.

Then solder locking layer 1210 is formed as described above, to expose at least the top ball 140. Then the structure is heated, e.g. in an oven and preferably in vacuum, to sublimate or evaporate at least part (possibly all) of the material 1250; then the oven temperature is increased to reflow the solder 140C (FIG. 17B), and the solder 140C sinks, leaving the holes 1230. Material 1250 is chosen such that at, above, or below the solder melting temperature the material 1250 turns into vapor either by sublimation, i.e. directly from the solid phase, or via melting (i.e. from the liquid phase). The melted or unmelted material 1250 may first flow to the top through the melted solder and then turn into vapor. Thus in some embodiments, the material 1250 has a lower density than solder 140C. An exemplary density of material 1250 is 0.6 to 3 kg/cm3. In some embodiments, the melted or unmelted material 1250 turns into vapor without first flowing to the top. As noted above, in some embodiments, the sublimation temperature of material 1250 is lower than the melting point of the solder 140C.

Material 1250 may or may not be electrically insulating, but the resulting solder connection is conductive because all or substantially all material 1250 is gone. In some embodiments, some material 1250 remains in the final structure, in amounts that do not impede electrically functionality of the final structure (possibly do not noticeably change the resistance of the solder connection).

Suitable materials 1250 are those that can function as described above. In particular, in some embodiments, material 1250 adheres well to core 140C, forming a coating of a predictable size. When heated to a suitable temperature, material 1250 of top ball 140 adheres to material 1250 of the ball immediately below, and adhesion remains good after cooling. In some embodiments, only one of the two balls is heated. In those embodiments in which a ball with material 1250 is placed on a lower ball's or contact pad's surface not covered by material 1250 (e.g. on an all-solder ball), the material 1250 is chosen to provide good adhesion with the surface of the lower ball. In some embodiments, good adhesion is obtained even in the presence of oxides on the ball being placed and the surface on which the ball is placed if such oxides can form before or during placement.

Suitable materials include the waxes, polyhydric alcohols, PEO, terpenoids, and other materials described above in connection with FIG. 16A. At least some of these materials have the suitably low density as described above, they can be made suitably soft for placement at a temperature below the melting temperature of many solders (e.g. at 120° C. for some materials), they can have low densities as specified above, and they can sublimate at fairly low temperatures, between 60 to 200° C. in some examples; such sublimation temperatures are suitable for use with solders having a melting point above 200° C. for example.

FIG. 17B shows squeeze-out portions 520 which are areas initially filled by material 1250 but later filled by solder 140C. The squeeze-outs may or may not be present.

Sidewall 1232 of layer 1210 at each hole 1230 may be concave; the sidewall completely laterally surrounds the connection. Each hole 1230 is a top portion of the through-hole in which the layer 1210 physically contacts the connection 210. The invention is not limited to such particulars.

In some embodiments, the amount of material 1250 is chosen to provide desired dimensions for the holes 1230. As the solder sinks during sublimation or evaporation of material 1250, some of the solder may remain on the sidewalls of solder locking layer 1210 to narrow the holes 1230. This narrowing can be minimal if the layer 1210 is not wetted by the solder. An exemplary depth of each hole 1230 is 1 to 200 microns and preferably 3 to 30 microns, and the hole's diameter at the top is 5 to 80% of the diameter of the corresponding connection 210. In each connection 210, the amount of material 1250 is about 3 to 50% by volume and preferably 5 to 25% by volume relative to the connection 210. These particulars are not limiting.

A single stack for a connection 210 may include any combination of balls 140 and balls of material 1250 of FIGS. 4A-17A. For example, in FIG. 18, each stack combines solder balls 140 as in FIG. 17A with a ball 1250 as in FIG. 16A. The top ball 140 includes a sublimatable or vaporizable coating of material 1250, and the middle ball of material 1250 is entirely sublimatable or vaporizable. Different sublimatable or vaporizable compounds or compositions can be used in different balls in the same stack (the term “composition” as used herein includes a pure compound). Material 1250 of all the balls is later sublimated or evaporated as described above, leaving holes 1230 as in FIG. 16B or 17B. Exemplary profiles and dimensions of holes 1230 and sidewalls 1232 can be as described above, but these profiles and dimensions are not limiting.

Of note, in some embodiments, some of the balls 140 are not melted. For example, referring to FIG. 10, if a ball 140 is attached to adjacent contact pads or balls by intermediate solder 140 i, then the ball 140 may be a high melting temperature material (possibly solder) or a non-melting material (such as a thermosetting polymer which decomposes at a high temperature rather than melt). The intermediate ball 140 i can be made of material 1250 in some embodiments, the balls above and below (or the ball above and the contact pad below) are fused together when ball 140 i sublimes or evaporates.

In some embodiments, the solder melting temperatures are such that even after the structure-to-structure attachment, all the connections can be undone by a conventional method, e.g. reflow in an oven, without damaging the structures. The oven reflow can be more convenient than the limited-spread radiation. The melting temperatures are under 450° C. in some embodiments.

The attachment methods described above can be used in many packaging processes, and possible benefits are illustrates in FIGS. 19A-19B. In FIG. 19A, a structure 110A is a wiring substrate, e.g. an interposer made using silicon and/or metal and/or rigid and/or flexible insulator and/or other material. Structure 110A has contact pads 120A.1 and smaller contact pads 120A.2 which are interconnected by a network of conductive interconnects 1310 as needed. A die (e.g. a semiconductor integrated circuit, e.g. a silicon circuit) 110B is flip-chip connected to contact pads 120A.2. In exemplary embodiments, die 110B is 200 μm thick; the structure 110A has 5 rows of contact pads 120A.1 at 240 μm pitch. The die connection can be by solder, thermocompression, or other techniques, which may or may not use some embodiments of this invention.

The die can be optionally underfilled (the underfill is not shown).

Solder may or may not have been deposited (e.g. electroplated or evaporated or printed) on pads 120A.1 before the die attachment. Then stacks 210 (FIG. 19B) are formed on contact pads 120A.1 by any method described above. In this illustration, each stack 210 has two solder balls jetted and fused together by partial melting techniques. The pre-melted solder-ball diameter is 175 μm, and the solder stacks rise higher than die 110B. In some embodiments, more than two solder balls (and even more than three solder balls) may be fused to provide the 175 micron height. The invention is not limited to the number of solder balls in each stack or to any particular height.

The number and diameter of the solder balls can be chosen as needed. (In one example, the pre-melted diameter is 25 μm, and each solder stack has more than 10 solder balls.) In some embodiments, all the bottom solder balls in each stack are formed before any overlying ball; in other embodiments, the entire stacks 210 are formed sequentially one after another.

Then solder locking layer 1210 can be formed by strip molding or film assist molding for example. Wet blasting with aluminum slurry can be used to expose the solder stacks 210 at the top. In an exemplary embodiment with more than two solder balls per stack, the height-to-pitch ratio of the solder stacks can be 2:1.

Another structure, e.g. an integrated circuit die or wafer or a packaging substrate or a package, can be attached to the tops of solder stacks 210. In the example of FIG. 20, the top structure includes a structure 110C (e.g. an integrated circuit or a wiring substrate) attached to an integrated circuit 110D formed as a die and flip-chip mounted on structure 110C. A molded compound (not shown) may then be provided to encapsulate the entire assembly on structure 110A. Structure 110C has contact pads 120C.2 attached to die 110D, and has contact pads 120C.1 bumped with solder balls 140.1 which are then soldered to stacks 210 formed on pads 120A.1 of structure 110A. Additionally or alternatively, posts or pins may be provided on the bottom of structure 110C for structural standoff between the structure 110C and the underlying package. In such an implementation, the posts or pins may be coated with solder 140.1. Moreover, the posts or pins may be configured to be inserted into some of the underlying solder stacks 210. When implemented with the techniques illustrated in FIGS. 14A through 18, such pins or posts may additionally assist with alignment of structure 110C to the underlying package.

In FIG. 20, solder 140.1 and solder stacks 210 were reflowed in an oven to attach the structure 110C to structure 110A; solder stacks 210 were constrained by solder locking encapsulant 1210 during the reflow. Interconnect lines 1310C in structure 110C connect the pads 120C.1 and the pads 120C.2 to each other as needed. In some embodiments, structure 110A includes a logic integrated circuit (e.g. a computer processor), and structure 110C includes a computer memory. This example is not limiting.

As illustrated, solder connections 210 may differ from each other: the leftmost connection 210.1 has small solder balls. The second stack 210.2 from the left has large solder-coated balls as in FIGS. 11A-11B (e.g. with a core made of copper or some other higher-melting-temperature material or a non-melting material, e.g. a cured polymer or an insulator). The remaining solder connections are made of large balls consisting entirely of solder. These examples are not limiting.

As noted above, use of the techniques described above in connection with FIGS. 4A-20 may increase the height-to-pitch ratio, which can be 0.8 or more per solder ball.

Some embodiments of the present invention facilitate manufacture of assemblies with different-length connections between structures. Such manufacturing capability is desirable to provide small size and short circuit paths in integrated circuit packages; see for example U.S. Pat. No. 8,618,659 issued Dec. 31, 2013 to Sato et al., incorporated herein by reference. An example package obtained in some embodiments of the present invention is shown in FIG. 21A. The package includes short connections 210.1 connecting a structure 110A to a structure 110B, and longer connections 210.2 connecting the structure 110A to a structure 110C. Each of structures 110A, 110B, 110C may be a packaged or unpackaged integrated circuit or a packaging substrate. Structure 110C may or may not be connected to structure 110B by additional connections. Each of such connections can be a solder ball, a solder stack, a solder wire, a wire bond (such as described in the aforementioned Sato et al. U.S. Pat. No. 8,618,659 or some other kind of wire). In particular, the connections can be as described above in relation to FIGS. 1A-20. These examples are not limiting.

Solder locking layer 1210 laterally surrounds each connection 210.1, 210.2. In some embodiments, solder locking layer 1210 is a molded layer (e.g. epoxy) made in a single molding operation in a specially designed mold. Alternatively, solder locking layer 1210 may be a combination of one or more thin films and a molded compound (such as 1210.1, 1210.2 in FIG. 14B); the molded compound laterally surrounds each short and long connection 210.1, 210.2; the molded layer is made in a single molding operation. In either case, the molded layer extends along the entire height of each connection 210 (except possibly near the contact pads). The molded layer may include an opening 1230 as described above in relation to FIGS. 14B-18.

This structure can be fabricated as follows. First, the short connections 210.1 and long connections 210.2 are formed on respective contact pads 120A.1, 120A.2 (FIG. 21B), by solder jetting, printing, wire bonding or any other suitable method. Then the molded portion of solder locking layer 1210 is formed in mold 1910 (FIG. 21B), by transfer molding for example. (In the embodiment shown, the entire layer 1210 is formed in this molding operation; in other embodiments, thin-film layers are formed before the molding operation.) The molding cavity 1920 of mold 1910 is taller over solder connections 210.2 than over solder connections 210.1, to provide a stepped profile for layer 1210. A molding compound is injected into the cavity and cured. (Exemplary molding compounds include thermoplastic or thermosetting compounds, e.g. resins (e.g. epoxy resins).) The resulting structure, taken out of the cavity, is shown in FIG. 21C. At this stage, layer 1210 covers the connections 210. Of note, layer 1210 does not have any interfaces typically present in a molded layer having multiple layers formed by respective multiple molding operations (the interfaces that separate the molded layers made in different molding operations). Layer 1210 is then polished using possibly standard techniques described above (e.g. dry or wet blasting, etc.). See FIG. 21D. The polishing operation may attack the interface 1924 of layer 1210 between the high connections 210.2 and low connections 210.1. This interface is formed by the mold cavity interface 1928 (FIG. 21B) between the high and low connections. In some embodiments, interface 1928 is a vertical wall, or is slightly sloped for ease of structure ejection after the molding step; and therefore layer 1210 has a vertical or near vertical step at interface 1924, but this vertical step becomes less vertical during polishing, e.g. the 90° angle may increase to, say, 100° or to some other value depending on the process parameters such as the material of layer 1210, the dimensions of various portions of layer 1210, and on the polishing operation.

Then structures 110B, 110C have their contact pads attached to respective connections 210.1, 210.2 using suitable techniques, e.g. as in prior art, to provide the assembly of FIG. 21A.

This technique can be extended to connections of three or more different heights. For example, a structure may have an array of two-ball solder stacks 210, another array of four-ball solder stacks, and a still other array of six-ball solder stacks, with all the solder balls of the same size in each stack. There may be additional arrays of still different connection heights of solder or non-solder connections. The molding cavity may correspondingly have three or more regions of different heights to create a corresponding stepped profile of molded layer 1210 in a single molding operation. All the connections can then be revealed at the top in a single polishing operation.

In solder stacks described above, balls 140 or 140 i or balls of material 1250 do not have to be stacked on top of one another: the balls may extend laterally or in any other direction to provide suitable interconnections. Further, the connections can fork; e.g. a single ball 140 may be connected to two or more other balls, of the same or smaller size, and each of the other balls may be part of a chain of one or more balls, each chain providing a separate prong of the fork. The prongs may arise from the same contact pad (similar to the wires in the aforementioned Sato et al. patent). The balls may be deposited by printing methods. For example, wax balls of material 1250 can be deposited by solid ink printers.

Some embodiments of the present invention provide a manufacturing method comprising fabricating a connection (e.g. a stack 210) which physically contacts a first conductive feature (e.g. a contact pad 120), the connection comprising a stack of members (e.g., each member can be a solder ball or a solder-coated ball), wherein fabricating the connection comprises performing a sequence of attaching operations each of which attaches a corresponding member (e.g. attaches a single ball 140) to a preceding component which is the first conductive feature or a preceding member of the stack;

wherein in at least one attaching operation, at least one of (A), (B) and (C) is true:

(A) the corresponding member's surface is a solder surface of which only a portion is melted in the attaching operation, the portion comprising a part which is fused with the preceding component in the attaching operation, the rest of the solder surface being unmelted in the attaching operation (see FIG. 7 for example);

(B) at least a part of the corresponding member is not melted in the attaching operation, and the preceding component is the preceding member whose surface is a solder surface of which only a portion is melted in the attaching operation, the melted portion comprising a part which is fused with the corresponding member in the attaching operation, the rest of the solder surface of the preceding member being unmelted in the attaching operation (see FIG. 6A for example);

(C) at least part of the corresponding member is not melted in the attaching operation and neither is at least part of the preceding component, but an intermediate member's solder surface (e.g. the surface of intermediate ball 140 i) is melted to fuse with the corresponding member and with the preceding component.

In some embodiments, the first conductive feature is a contact pad of a semiconductor integrated circuit (e.g. a die or a wafer) or of a substrate (e.g. an interposer or a printed circuit board) attachable to a semiconductor integrated circuit.

In some embodiments, the method further comprises, after forming the connection, attaching the connection to a second conductive feature (e.g. another pad 120) which is a contact pad of a semiconductor integrated circuit or of a substrate attachable to a semiconductor integrated circuit.

In some embodiments, at least one of (A), (B) and (C) is true in each attaching operation.

In some embodiments, the melting is performed by a beam of electromagnetic radiation.

In some embodiments, at least one of (A) and (B) is true, and for each solder surface having a melted portion, the solder surface is continuous and all of the solder surface has the same melting temperature. For example, the entire solder surface can be the same material.

In some embodiments, (C) is true, and the intermediate member's pre-melted volume is at most 100% of the corresponding component's pre-melted volume.

In some embodiments, at least one of (1) and (2) is true:

(1) statement (A) is true, the corresponding member has a core that does not melt at a melting temperature of the solder melted in the attaching operation, and the core is not melted in the attaching operation;

(2) statement (B) is true, the preceding component is a preceding member having a core that does not melt at a melting temperature of the solder melted in the attaching operation, and the core is not melted in the attaching operation.

In some embodiments, the core physically contacts the melted solder in the attaching operation. For example, in FIG. 11A, the melted portion 510 of shell 140S may reach the core 140C.

Some embodiments provide a method for attaching a first conductive component (e.g. a ball 140) to a second conductive component (e.g. another ball 140 or a contact pad 120), the method comprising:

(1) melting a first portion of a surface of the first conductive component without melting a second portion (e.g. the rest) of the surface of the first conductive component even though the second portion's melting temperature is no higher than the first portion's melting temperature, wherein the second portion is continuous with the first portion (e.g. as in an all-solder ball 140 or a shell 140S);

(2) bringing the melted first portion into physical contact with the second conductive component and cooling the first portion to form a conductive attachment between the first and second conductive components.

In some embodiments, in operation (2), the melted first portion is brought into physical contact with the second conductive component when the second conductive component is unmelted.

In some embodiments, a maximum pre-melted dimension of the first portion does not exceed 700 μm.

In some embodiments, the first portion is melted to a depth not exceeding 500 μm.

In some embodiments, the pre-melted volume of a melted portion of the first conductive component does not exceed 50% of the first conductive component.

In some embodiments, the first conductive component comprises a core having a higher melting temperature than the first portion of the surface of the first conductive component.

Some embodiments provide a method for fabricating an electrical connection, the method comprising:

providing a first structure which comprises a first contact pad (e.g. 120 in FIG. 9 or 11A);

forming a first portion of the electrical connection on the first contact pad, the first portion comprising a first component comprising a first surface (e.g. the first component can be the ball 140.1 in FIG. 9 or 11A);

providing a second component of the electrical connection (e.g. ball 140 i), the second component comprising a second surface comprising solder;

providing a third component of the electrical connection (e.g. 140.2), the third component comprising a third surface;

attaching the second component to the first and third surfaces, wherein said attaching comprises melting and re-solidifying the solder to attach the solder to the first and third surfaces, wherein at least one of the first and third surfaces has an unmelted area which is not melted in said attaching even though a melting temperature of the unmelted area is not higher than a highest temperature of the solder in said attaching.

In some embodiments, the melting is performed only once during said attaching.

Some embodiments provide a structure comprising circuitry comprising a semiconductor integrated circuit, the circuitry comprising a conductive connecting feature (e.g. a stack 210 in FIG. 12B or 22) which has a first end (e.g. 210E.1 in FIG. 22) and a second end (e.g. 210E.2), the connecting feature extending along a first line (e.g. 1504; the line does not have to be vertical and can be curved) from the first end to the second end, wherein at each point of the first line, the connecting feature has a transversal cross-sectional area (e.g. circular area A at point P in FIG. 22) which is an area of the connecting feature's cross section perpendicular to the first line at said point;

wherein the connecting feature comprises a first segment (e.g. 1510 in FIG. 22, or the bottom half of ball 140.1 in FIG. 12B; the first segment can be the ball's bottom portion less than half a ball in FIG. 12B), a second segment (e.g. 1520, which is squeeze-out 520.2) physically contacting the first segment, and a third segment (e.g. 1530, or the top half (or less than a half) of ball 140.2) physically contacting the second segment;

wherein the first segment has a first end (e.g. 1510E.1) and a second end (e.g. 1510E.2) which is located at a junction with the second segment;

wherein the third segment has a first end (e.g. 1530E.1) which is located at a junction with the second segment, and the third segment has a second end (e.g. 1530E.2);

wherein the transversal cross-sectional area:

-   -   decreases from the first end of the first segment to the         junction of the first and second segments;     -   increases from the junction of the first and second segments to         a position (e.g. 1520P, the squeeze-out middle) in the second         segment;     -   decreases from said position in the second segment to a junction         of the second and third segments; and     -   increases from the junction of the second and third segments to         the second end of the third segment.

In some embodiments, surfaces of the first, second and third segments have the same melting temperature.

In some embodiments, a diameter of the transversal cross-sectional area:

-   -   decreases from the first end of the first segment to the         junction of the first and second segments;     -   increases from the junction of the first and second segments to         a position (e.g. 1520P, the squeeze-out middle) in the second         segment;     -   decreases from said position in the second segment to a junction         of the second and third segments; and     -   increases from the junction of the second and third segments to         the second end of the third segment.

In some embodiments, in any view in which the second end lies directly above the first end (for example, in the view of FIG. 12B or 22, or any side view (e.g. the solder stack can be symmetric about the vertical axis)), the conductive connecting feature's width (e.g. the diameter D or area A):

-   -   decreases from the first end of the first segment to the         junction of the first and second segments;     -   increases from the junction of the first and second segments to         a position (e.g. 1520P, the squeeze-out middle) in the second         segment;     -   decreases from said position in the second segment to a junction         of the second and third segments; and     -   increases from the junction of the second and third segments to         the second end of the third segment.

In some embodiments, the length of the second segment (the squeeze-out height) is at most 50% of the length of each of the first and third segments (e.g. of the length of one half of each of the balls). The length of the second segment can be even smaller, e.g. 1% or 2% of the length of each of the first and third segments. (For each numerical range described in this disclosure, the invention covers all the possible sub-ranges and any values in the range; thus, any sub-range or value of above 0% and less or equal to 50% can be present.)

In some embodiments, the volume of the second segment is at most 50% of the volume of each of the first and third segments.

Some embodiments provide a structure comprising circuitry comprising a semiconductor integrated circuit, the circuitry comprising a conductive connecting feature (e.g. solder ball 140 or stack 210 in FIG. 4B or 5B or 6B or 11B or 12B or 17B or 23) which has a first end (e.g. 210E.1 in FIG. 23) attached to a contact pad, the connecting feature having a second end, the connecting feature extending along a first line (e.g. 1504; the line does not have to be vertical and can be curved) from the first end to the second end, wherein at each point of the first line, the connecting feature has a transversal cross-sectional area which is an area of the connecting feature's cross section perpendicular to the first line at said point;

wherein at least one of the transversal cross-sectional area and a diameter of the transversal cross-sectional area:

-   -   decreases from the contact pad to a first point in the first         line (for example, the first point can be the bottom end 1610 of         squeeze-out 520 in FIG. 4B or 17B or 23); and     -   increases from the first point to a second point in the first         line (e.g., the second point can be the middle 1620 of ball 140         in FIG. 4B or 23, or can be below the middle of the ball);

wherein the distance, along the first line, from the contact pad to the first point (e.g. the squeeze-out height D1) is at most 25% of the distance between the first point and the second point (e.g. distance D2, which is less than or equal to half the height of ball 140). As noted above, the invention covers all sub-ranges from above 0% to less than or equal to 25%, and all values in the sub-ranges, including for example 1% and 2% as possible values of D1 as percentage of D2.

Some embodiments provide a structure comprising circuitry comprising a semiconductor integrated circuit, the circuitry comprising a conductive connecting feature (e.g. solder ball 140 or stack 210 in FIG. 4B or 5B or 6B or 11B or 12B or 17B or 23) which has a first end (e.g. 210E.1 in FIG. 23) attached to a contact pad, the connecting feature having a second end, the connecting feature extending along a first line (e.g. 1504; the line does not have to be vertical and can be curved) from the first end to the second end, wherein at each point of the first line, the connecting feature has a transversal cross-sectional area which is an area of the connecting feature's cross section perpendicular to the first line at said point;

wherein in any side view, the transversal cross-sectional area's width:

-   -   decreases from the contact pad to a first point in the first         line; and     -   increases from the first point to a second point in the first         line;

wherein the distance, along the first line, from the contact pad to the first point is at most 25% of the distance between the first point and the second point.

Some embodiments provide a first structure comprising circuitry comprising a semiconductor integrated circuit, the circuitry comprising a conductive connecting feature for connecting a contact pad of the first structure to another structure, the conductive connecting feature comprising a first end attached to the contact pad and comprising a second end for being attached to the other structure;

wherein the connecting feature comprises a first non-solder ball and a second non-solder ball (e.g. as in FIG. 11B) that are positioned sequentially between the first and second ends one after another along the connecting feature's path from the first end to the second end;

wherein the connecting feature has a continuous solder surface which laterally surrounds the first and second non-solder balls;

wherein the continuous solder surface has a continuous protrusion adjacent to the first and second balls, wherein the continuous protrusion protrudes laterally on all sides of the connecting feature. The first and second balls may or may not touch each other. In some embodiments, the distance between the first and second balls is at most 1 mm.

Some embodiments provide a manufacture comprising:

a first structure comprising a contact pad;

an electrically conductive connection having a bottom attached to the contact pad; and

a dielectric layer (e.g. 1210 in FIGS. 14B, 16B, 17B) comprising a top surface and a through-hole in the top surface, the through-hole containing at least a segment of the electrically conductive connection, said segment comprising solder and physically contacting the dielectric layer in the through-hole, the electrically conductive connection being recessed in the through-hole at the top surface of the dielectric layer. Of note, the through-hole and the connection may or may not be vertical.

In some embodiments, the through-hole has a concave sidewall at the top surface of the dielectric layer.

In some embodiments, the electrically conductive connection is attached to a conductive post (e.g. 1240) of a second structure, the conductive post being partially located in a top portion of the through-hole.

The invention is not limited to the embodiments described above. Other embodiments and variations are within the scope of the invention, as defined by the appended claims. 

The invention claimed is:
 1. A method for attaching a first conductive component to a second conductive component, the method comprising: (1) melting a first portion of a surface of the first conductive component without melting a second portion of the surface of the first conductive component even though the second portion's melting temperature is no higher than the first portion's melting temperature, wherein the second portion is continuous with the first portion; (2) bringing the melted first portion into physical contact with the second conductive component and cooling the first portion to form a conductive attachment between the first and second conductive components; wherein the second portion is not melted in operations (1) and (2); and wherein the first conductive component comprises a core having a higher melting temperature than the first portion of the surface of the first conductive component.
 2. The method of claim 1 wherein in operation (2), the melted first portion is brought into physical contact with the second conductive component when the second conductive component is unmelted.
 3. The method of claim 1 wherein a maximum pre-melted dimension of the first portion does not exceed 700 μm.
 4. The method of claim 1 wherein the first portion is melted to a depth not exceeding 500 μm.
 5. The method of claim 1 wherein the pre-melted volume of a melted portion of the first conductive component does not exceed 50% of the first conductive component.
 6. The method of claim 1 wherein the second conductive component is a contact pad of a semiconductor integrated circuit or of a substrate attachable to a semiconductor integrated circuit, or the second conductive component is a connection component that has been attached to such contact pad before operation (1).
 7. The method of claim 1 wherein the melting is performed by a beam of electromagnetic radiation.
 8. A method for fabricating an electrical connection, the method comprising: providing a first structure which comprises a first contact pad; forming a first portion of the electrical connection on the first contact pad, the first portion comprising a first component comprising a first surface; providing a second component of the electrical connection, the second component comprising a second surface comprising solder; providing a third component of the electrical connection, the third component comprising a third surface; attaching the second component to the first and third surfaces, wherein said attaching comprises melting the solder to obtain melted solder in physical contact with the first and third surfaces, and re-solidifying the solder to attach the solder to the first and third surfaces, wherein at least one of the first and third surfaces has an unmelted area which is not melted in said attaching even though a melting temperature of the unmelted area is not higher than a highest temperature of the solder in said attaching.
 9. The method of claim 8 wherein the first and third surfaces are not melted in said attaching.
 10. The method of claim 8 wherein at least one of the first and third surfaces has the same melting temperature as the solder.
 11. The method of claim 8 wherein the third component is a solder ball which has the same melting temperature as the second component's solder but which is not melted in said attaching.
 12. The method of claim 8 wherein the first structure comprises at least one of a semiconductor integrated circuit or a substrate attachable to a semiconductor integrated circuit.
 13. The method of claim 8 wherein the melting is performed by a beam of electromagnetic radiation.
 14. The method of claim 8 wherein the solder surrounds a core that does not melt in said attaching.
 15. The method of claim 8 wherein the melting is performed only once during said attaching.
 16. The method of claim 1 wherein the first conductive component does not physically contact the second conductive component during operation (1).
 17. The method of claim 1 wherein the entire surface of the first component is the same material.
 18. The method of claim 7 wherein the beam of electromagnetic radiation arrives at the first portion without first passing through the second portion.
 19. The method of claim 7 wherein the beam of electromagnetic radiation arrives at the first portion without first passing through the first conductive component.
 20. The method of claim 8 wherein the unmelted area physically contacts the melted solder in said attaching. 